p53 mutated in the transactivation domain retains regulatory functions in homology-directed double-strand break repair

The tumor suppressor p53 transcriptionally transactivates cellular target genes that are implicated in growth control, apoptosis, and DNA repair. However, several studies involving p53 core domain mutants suggested that regulatory functions in recombinative repair do not require transcriptional transactivation and are separable from growth-regulation and apoptosis. Leu22 and Trp23 within the transactivation domain of human p53 play a critical role in binding basal components of the transcription machinery and, therefore, in the transactivation activity of p53. To further delineate whether p53 target genes are involved in recombination regulation, we ectopically expressed p53(22Q,23S) in p53-negative cell lines, which carry reporter systems for different homology-directed double-strand break (DSB) repair events. Like wild-type p53, p53(22Q,23S) efficiently downregulated homologous recombination on two chromosomally integrated substrates without affecting exchange on a substrate for the compound pathway of gene conversion and nonhomologous end joining. Only upon lowering the p53 protein to DNA substrate ratio by several orders of magnitude, we noticed a weak defect of a p53 transactivation domain mutant in DSB repair assays. In conclusion, molecular interactions of p53 within the N-terminal domain are not required to restrain DNA recombination, but might contribute to this genome stabilizing function.